Lead-free phosphate glasses

ABSTRACT

Disclosed are zinc niobium phosphate glasses consisting essentially, expressed in terms of mole percent on the oxide basis of, 40-65% P2O5, 25-37% ZnO, 0.1-15% Nb2O5, 0-6% Al2O3, 0-5% Bi2O3, 0-3% Na2O and 0-5% B2O3, said glass exhibiting glass transition temperature below 450° C., a dilatometer softening point below 500° C., coefficient of expansion in the range of 80-120×10 −7 ° C. −1 , good chemical durability and blue color. The glass of the present invention is useful for sealing optical fiber to ferrule by local heating.

FIELD OF THE INVENTION

This invention relates to low glass transition temperature zincphosphate glasses. More particularly, the present invention relates tolead-free zinc phosphate glass composition with low glass transitiontemperature.

BACKGROUND OF THE INVENTION

Glass in the form of powder (obtained by grinding glass frits) have beenused for many years as sealants and as coating on glasses, ceramics,metals and in thick film microelectronics. To function as a lowtemperature sealant and as an overglaze in thick film microelectronics,the glass has to soften and flow at temperatures lower than 500° C. Mostof the commercial compositions are based on lead borate (PbO—B₂O₃)system or its modifications with zinc (PbO—B₂O₃—ZnO). These glassescontain high levels of PbO (typically 70-80 wt %) and have lowdurability. Lead is toxic and the trend today is to limit its use andeventually eliminate it from compositions used in sealing andmicroelectronics. Most of the devices made with high lead containingglasses find their way to dump sites where the lead is leached into thewater table, the leaching is facilitated because of the low durabilityof high lead containing glasses.

One object of the present invention was to devise lead-free glasscompositions having a glass transition temperature (Tg) no higher than450° C. and, in a form of powders, would demonstrate flow propertiessimilar to those displayed by the above mentioned lead glasses.

Another object of the present invention is to provide glass compositionswith sufficient durabilities, expansion coefficients and flow propertiesfor sealing optical fibers to ceramic ferrules and Kovar™.

Yet another object of the present invention is to provide glasscompositions, essentially lead-free, and having absorption bands in thevisible and/or in the near infra red region of the spectrum to allowlocal heating (and hence sealing) by laser radiation.

SUMMARY OF THE INVENTION

It has now been found by the inventor that by appropriately choosing theingredients of the glass composition and controlling theirconcentrations within well-specified ranges, it is possible to obtain alow softening point zinc phosphate glass which is free of lead, and yetpossesses valuable physical and chemical properties which render theglass useful for various purposes, in particular, as a sealant ofoptical fiber to a ferrule made of ceramics or Kovar.

Thus, according to one aspect of the invention, there is provided aglass composition consisting, in terms of mole percent of the totalcomposition, 40.1-75% glass forming compounds and the balance glassmodifiers. The 40.1-75% glass forming compounds consists of 40-65% P2O5;0.1-15% Nb2O5 and the glass modifiers consists of 25-37% of ZnO; 0-6%Al2O3; 0-10% Bi2O3, 0-5% B2O3; Ta2O5, V2O5, Sb2O3, ZrO2, and/or TiO2 intotal amount of 0-5%; 0-3% alkali oxides; 0-5% alkali fluorides; oxidesof Mg, Ca, Sr, Ba, Cu, Fe, and Mn, in a total amount of 0-5%.

Hereinafter, by the term “glass-forming compounds” is meant the class ofcompounds each of which is capable of forming a glass by itself uponmelting and cooling, as well as the class of compounds which are usuallyknown in the art as “conditional glass-forming compounds”. Conditionalglass-forming compounds do not form a glass by themselves upon meltingand cooling, however, they form a glass when mixed with anotherappropriate compound. SiO₂, P₂O₅ and B₂O₃ are compounds belonging to thefirst class, whereas Bi₂O₃, Al₂O₃ and Nb₂O₅ are members of the secondclass.

Suitable glass forming compounds as defined above, in addition to P₂O₅,B₂O₃, Bi₂O₃, Al₂O₃ and Nb₂O₅, may be selected, for example, from amongthe group consisting of Ta₂O₅, V₂O₅, Sb₂O₃, ZrO₂ and TiO₂ or mixturesthereof. Among these compounds, most preferred are Ta₂O₅ and V₂O₅, inthe amount of at most 5 mole %, TiO₂ and/or ZrO₂, the sum of the amountsof said TiO₂ and/or ZrO₂ being at most 3 mole % of the totalcomposition. The total amount of all glass forming compounds, i.e.,P₂O₅, Nb₂O₅, Al₂O₃ and Bi₂O₃ and the optional additional glass formingcompounds, selected from the group consisting of B₂O₃, Ta₂O₅, V₂O₅,Sb₂O₃, ZrO₂ and TiO₂ or mixtures thereof, is 40.1-75 mole %.

The term “glass modifiers” is herein used to define a class of oxidesand halogen compounds which do not possess the ability to form a glassby themselves upon melting and cooling, nor when contacted withadditional compounds, other than compounds belonging to the class ofglass forming compounds described above. However, the glass modifiersare capable of modifying the chemical and physical properties of theglasses containing them. ZnO and Na₂O are known in the art as glassmodifiers. Suitable glass modifiers which are optionally included in theglass compositions according to the present invention, in addition toZnO and Na₂O may be selected for example from the group consisting ofMgO, CaO, SrO, BaO, oxides and/or fluorides of alkali metals and oxidesof transition metals such as CuO, MnO, and Fe₂O₃. The total amount ofall glass modifiers present in the composition, i.e., ZnO, alkalioxides, alkali fluorides, MgO, CaO, SrO, BaO, and oxides of transitionmetals such as CuO, MnO and Fe₂O₃, is 25-59.9 mole % of the totalcomposition.

The above definitions of the terms “glass-forming” compounds” and “glassmodifiers” are based on the general properties of the compounds as theyare known and accepted in the art, without being bound to anyexplanation relative to the exact role of each of these compounds in thecompositions according to the present invention.

According to another aspect of the present invention, there is provideda glass composition that is free of lead and cadmium, has a T_(g) ofbetween 200° C. and 500° C., and expansion coefficient of 12×10⁻⁶K⁻¹ orless, said glass comprising, by mol percent of the total composition, atleast 3% Nb₂O₅ and/or Al2O3, 25-40% ZnO, and at least 40% P₂O₅.

Another aspect of the present invention relates to thick filmformulations comprising finely divided particles of the glass of thepresent invention dispersed in an organic medium, optionally togetherwith other solids, such as, for example, conductive metal particles,metallic oxide or ceramic fillers, to various applications of saidformulations and to an article comprising said formulation fired on aceramic substrate.

RELATED ART

Modifications of zinc phosphate glasses were the theme of many previousUS patents and publications in journals. N. H. Ray et al. “Oxide glassesof very low softening point 2. Preparation and properties of some zincphosphate glasses”, Glass Technology (1973), 14 (2) 55-9, describe zincphosphate glasses containing alkali and alkaline earth metal oxides withglass transition temperature (Tg) as low as 150° C. This paper alsopresents the water resistance of these glasses, viscosities and theeffects of Li₂O and MgO on the properties. Beall et al. in U.S. Pat.Nos. 4,940,677; 5,071,795; 4,996,172; 5,122,484 and 5,328,874 teacheszinc phosphate low glass transition temperature glasses and reviews theprior art relating to glasses exhibiting transition temperatures (Tg)below 450° C. coupled with good resistance to attack by water. Glassestaught by Beall are multicomponent glasses which are lead-free (U.S.Pat. Nos. 5,122,484 ; 5,328,874 and 5,071,795) or incorporate lead (U.S.Pat. Nos. 4,996,172 and 4,940,677).

Y. He and D. E. Day in “Glass technology Vol. 33 No. 6 pp 214-219(1992),” reported the development of low temperature phosphate glasses.Their study shows that phosphate glasses with very good durability (inboiling water) can be synthesized with proper choice of the oxides.

V. Akishin et al. “Low melting phosphate glasses to thick filmresistors”, Phosphorus Research Bulletin (2002), 13 , 73-76, teaches lowmelting zinc metaphosphate glasses (ZnO:P₂O₅=1:1, SiO₂ 8-12, Al₂O₃2-8mole %) for thick film resistors.

M. Onosawa in Japanese patent No. JP 2000072473 issued Mar. 7, 2000(Application No. JP 1998-248545) teaches low melting point bismuth zincphosphate glass and sealing compositions. The glass contains (wt %oxides) 45-70 P₂O₅, 5-30 ZnO, 5-30 Bi₂O₃, 1-10 CuO, 0-10 Sb₂O₃, 0-10V₂O₅, 0-10 MnO₂, 0-10 FeO, where CuO+Sb₂O₃+V₂O₅+MnO₂+FeO 2-20, 0-5 SiO₂,0-5 B₂O₃, 0-5 Al₂O₃ and 0-5 La₂O₃. The sealing compositions may contain58-95 wt % the glass and 5-42 wt % refractory fillers free from Pb andalkali metals.

V. Akishin et al. “Low melting lead free phosphate glasses forelectronics”, Latvijas Kimijas Zurnals (1997), (2), 101-106, reports lowmelting (Tg=360-420° C.) lead free glasses on Zn metaphosphate (mole %:ZnO=P₂O₅ 36-44; SiO₂ 8-12 V₂O₅ 0-10; Al₂O₃2-8).

J. Chiba in Japanese patent JP09188544 issued Jul. 22, 1997 (applicationNo. JP 1996-2581) “Lead free weather resistant low melting zincphosphate based sealing glass compositions”, teaches glass compositionscomprising (wt %): ZnO 15-55, P₂O₅ 30-75, Li₂O+Na₂O+K₂O 0-2.8, Al₂O₃0.1-5, SnO₂+TiO₂+ZrO₂ 0.1-2, Bi₂O₃ 0-30, SiO₂ 0-5, B₂O₃ 0-10, V₂O₅+Ti₂O₃0-5 and F 0-2 wt %.

B. G. Aitken U.S. Pat. No. 5,286,683 teaches low transition temperatureglass <375° C. which is resistant to boiling water. His most preferredglass composition comprises, Li₂O 7.0, Na₂O 8.0, K₂O 5.0, CuO 15.0, ZnO0.9, ZnF₂ 14.1, CaF₂ 7.5, BaF₂ 7.5, Al₂O₃ 2.0, P₂O₅ 33.0 mole %. Thiscomposition contains high concentration of alkali metals oxides (20 mole%), high concentration of fluorides and low concentration of ZnO andP₂O₅.

DETAILED DESCRIPTION OF THE INVENTION

In the discussion which follows, all compositional percentages are mole%, unless otherwise stated, and whenever compositional ranges areindicated, they should be understood as including the lower and upperlimits.

The invention provides a glass composition comprising, in mole percentof the total composition: Glass-forming compounds in a total amount of40.1-75%, wherein said glass forming compounds comprise 40-65% P₂O₅,0.1-15% Nb₂O₅, 0-6% Al₂O₃, 0-10% Bi₂O₃ and 0-5% B₂O₃, and glassmodifiers in a total amount of 25-59.9%, wherein said glass modifierscomprise 25-37% ZnO and 0-3% Na₂O.

It has been found by the inventor that the amount of P₂O₅ in the glassshould be at least 50 mole % of the total composition in order to renderthe viscosity of the glass suitable for sealant applications and toprovide the appropriate range for the formation of blue color. It hasalso been found by the inventor that the amount of P₂O₅ must not exceedthe limit of 65 mole %, since higher amounts might lead tounsatisfactory durability.

The range of amounts of the Nb₂O₅ according to the present invention isbetween 0.1-15 mole % of the total composition. Within that range, theviscosity and durability of some compositions are suitable forapplications as sealant and thick film formulations. In this compositionrange an intense blue color is developed. Preferably, the amount ofNb₂O₅ is between 0.1-12.5 mole %, more preferably 1-10 mole %.

The range of the amounts of the Al₂O₃ according to the presentinvention, is between 0-6 mole % of the glass composition. Within thatrange, the Al₂O₃ has beneficial effect on the durability, glass-formingability and thermal expansion of the glass. Preferably, the amount ofAl₂O₃ will be between 0.1 -5 mole %, most preferably around 2 mole %.The range of the amount of the Bi₂O₃ according to the present inventionis between 0-10 mole % of the total composition inclusive. Within thatrange, the viscosity and durability of the glass are suitable forapplications as sealant and in thick film formulations. Preferably, theamount of Bi₂O₃ is between 0.1-5 mole %.

According to a preferred embodiment of the present invention, the glasscomposition may optionally comprise glass-forming compounds in additionto P₂O₅, Nb2O₅, Al₂O₃ and Bi₂O₃, wherein the total amount of said P₂O₅,Nb₂O₅, Al₂O₃ and Bi₂O₃ and said optional glass forming compounds beingbetween 40.1-75 mole % of total composition. Suitable glass formingcompounds as defined above, in addition to P₂O₅, Nb₂O₅, Al₂O₃ and Bi₂O₃may be selected, for example, from the group consisting of B₂O₃, Ta₂O₅,V₂O₅, Sb₂O₃, ZrO₂ and TiO₂ or mixtures thereof.

In another preferred embodiment, the present invention provides a glasscompositions comprising, in mole % of the total composition:glass-forming compounds in total amount of 40.1 to 75 mole %, whereinsaid glass forming compounds comprise 40-65 mole % P₂O₅, 0.1 -10 mole %Nb₂O₅, 0.1-6 mole % Al₂O₃ and 0.1-10 mole % Bi₂O₃, and the glassmodifiers in total amount of 25 to 59.9 mole %, wherein said glassmodifiers comprise 25-37 mole % ZnO and 0-3 mole % Na₂O.

ZnO and Na₂O are considered in the art as glass modifiers. The glasscomposition according to the present invention may include additionalglass modifiers which may be selected, for example, from the groupconsisting of MgO, CaO, SrO, BaO, oxides and/or fluorides of alkalimetals and oxides of transition metals such as CuO, MnO and Fe₂O₃, ormixtures thereof.

The above mentioned glass ingredients alter the properties of the glasscomposition in various aspects as will now be illustrated.

Preparation of the Glass:

The preparation of the glasses of the present invention can be carriedout by conventional methods which are well known in the art. Forexample, one such method consists of mixing together the desiredproportions of oxide or fluoride precursors, melting the mixture andpouring the molten composition into water to form a frit. An oxide orfluoride precursor may, of course, be any compound that will yield thedesired oxide or fluoride under the usual conditions of frit production.For example, boric oxide will be obtained from boric acid; P₂O₅ will beobtained from phosphoric acid, NH₄H₂PO₄ or from phosphates of zinc,sodium, aluminum and boron; barium oxide will be produced from bariumcarbonate; etc. The glass is preferably milled in a ball mill with waterto reduce the particle size. As is well known in the art, melting isconducted at a peak temperature and for a time such that the meltbecomes entirely liquid and homogeneous. In preparing the glasses of thepresent invention, the components are premixed by shaking inpolyethylene jar with plastic balls, and are then melted in a platinumor a high purity alumina crucible at the desired temperature. The meltis maintained at a peak temperature of 900-1300° C. for a period of10-30 minutes. The melt is then poured into cold water. The maximumtemperature of the water during quenching is kept as low as possible byincreasing the volume of water to melt ratio. The crude frit afterseparation from water is freed from residual water by drying in air orby displacing the water by rinsing with methanol. The crude frit is thenball milled for 3-5 hours in alumina containers using alumina balls.Alumina picked up by the materials, if any, is not within the observablelimit as measured by X-ray diffraction analysis. After discharging themilled slurry from the frit, the powder is air-dried. The dried powderis then screened through a 325 mesh screen to remove any largeparticles.

Another aspect according to the present invention is related to thickfilm formulations comprising finely divided particles of the glassaccording to the present invention dispersed, optionally together withadditional solids selected according to the intended use of theformulation, in an organic medium, conductive metal particles, the totalcomposition of the solids dispersed in the organic medium being between0.5 to 80 weight % glass particles and between 20 to 99.5 weight %conductive metal or conducting metallic oxide.

In another preferred embodiment of the present invention, the thick filmformulation comprises additional solids which are ceramic fillers, theweight of said ceramic fillers being not higher than 20% of the totalweight of solids dispersed in the organic medium. The ceramic fillersare, for example, selected from among vitreous silica, zircon andmixtures thereof.

Any inert liquid, such as various organic liquids, with or withoutthickening and/or stabilizing agents and/or other common additives, maybe used as the medium. Suitable organic liquids are aliphatic alcoholsor esters thereof, terpens such as pine oil, terpineol and the like,solutions of resins such as the polymethylacrylates of lower alcoholsand solutions of ethyl cellulose in solvents such as pine oil and themonobutyl ether of ethylene glycol monoacetate. Preferred organicmediums are ethyl cellulose solution in terpineol and butyl ethers ofethylene glycol.

Prepartation of Thick Film Formulations:

The preparation of thick film formulations according to the presentinvention can be carried out by conventional methods which are wellknown in the art. In the preparation of the formulations of the presentinvention the particulate inorganic solids are mixed with the organicmedium and dispersed with suitable equipment, such as a Muller, to forma suspension, resulting in a composition for which the viscosity will bein the range of about 100-150 pascalseconds at a shear rate of 4 sec⁻¹.

The thick film formulation according to present invention was preparedin the following manner: The organic mediums used consisted of a mixtureof diethylene glycol dibutyl ether, terpineol, and ethyl cellulose. Theingredients of the formulation, except for about 5 weight percent of theorganic components, are weighed together in a container. The componentsare then vigorously mixed to form a uniform blend; then the blend ispassed through dispersing equipment, such as a Muller, to achieve a gooddispersion of particles. A Hegman gauge is used to determine the stateof dispersion of the particles in the paste. This instrument consists ofa channel in a block of steel that is 25 μm deep (1 mil) on one end andramps up to 0″ depth at the other end. A blade used to draw down pastealong the length the length of the channel. Scratches will appear in thechannel where the agglomerates' diameter is greater than the channeldepth. A satisfactory dispersion will give fourth scratch point of 10-18μm, typically. A fourth scratch measurement of >10 μm indicates a poorlydispersed suspension. The remaining 5% consisting of organic componentsof the paste is then added, and the resin content is adjusted to bringthe viscosity, when fully formulated, to between 140 to 200 Pa. sec at ashear rate of 4 sec⁻¹. the composition is then applied to a substrate,such as alumina ceramic, usually by the process of screen printing, to awet thickness of about 30-80 microns, preferably 35-70 microns, and mostpreferably, 40-50 microns. The compositions of this invention can beprinted onto the substrates either by using an automatic printer or ahand printer in the conventional manner, preferably employing automaticscreen stencil techniques, using a 200 to 325 mesh screen. The printedpattern is then dried at below 200° C., about 150° C., for 5-15 minutesbefore firing. Firing to effect sintering of both the inorganic binderand the finely divide particles of metal is preferably done in a wellventilated belt conveyor furnace with a temperature profile that willallow burnout of the organic matter at about 300° C. C.-650° C., lastingabout 5-15 minutes, followed by a controlled cool-down cycle to preventover-sintering, unwanted chemical reactions at intermediatetemperatures, or substrate fracture, which can occur from a too-rapidcool-down. The overall firing procedure will preferably extend over aperiod of about 1 hour, with 20-25 minutes to reach the firingtemperature, about 10 minutes at the firing temperature, and about 20-25minutes to cool down. In some instances, total cycle times as short as30 minutes can be used.

The following examples are given for the purpose of illustration, andare not intended to limit the scope of the invention.

EXAMPLES Examples 1-11

The compositions of 11 glasses (in mole %) and some physical propertiesof interest: glass transition temperature (Tg), dilatometer softeningtemperature (Td) and linear coefficient of expansion in the temperaturerange of 100 to 300° C. (except some glasses with low Tg such asexamples No. 1,5 and 9 in table 1) are detailed in table 1. The sourceof P₂O₅ was H₃PO₄ for examples 1 to 11 and the melting was done inplatinum crucible. Table 1 shows that a simple zinc phosphate glass(example No. 1) has very low Td and high coefficient of expansion. Thisglass is very hygroscopic, it absorbs water from the atmosphere anddissolves. This glass has very poor durability. Example 2 to 8illustrate stable glasses with good durability (when B₂O₃ concentrationis ≧10 mole %) and small expansion. However, these glasses have high Tgand even with partial substitution of fluorides for ZnO (examples 6 to8) there is slight decrease in Tg but these glasses crystallize easily.Examples 9 to 11 illustrate the effect of Nb₂O₅: low Tg and Td anddurability comparable to the zinc borophosphate glasses. All the Nb₂O₅containing glasses have blue color.

TABLE 1 glass composition in mole % examples No. 1 1* 2* 3* 4* 5* 6* 7*8* 9 10 11 ZnO 30 30 30 30 30 25 20 25 30 30 30 P₂O₅ 70 50 60 65 67.5 5050 50 65 62.5 60 B₂O₃ — 20 10 5 2.5 20 20 20 — — — Nb₂O₅ — — — — — — — —5 7.5 10 Bi₂O₃ — — — — — — — — — — — Na₂O — — — — — — — — — — — Al₂O₃ —— — — — — — — — — — ZnF₂ — — — — — 5 10 — — — — LiF — — — — — — — 5 — —— Tg (° C.) — 593 428 345 281 531 544 515 283 381 491 Td (° C.) 148 642482 410 332 618 603 570 348 432 539 α₁₀₀₋ 141.1** 71.1 91.5 91.0 106.4+62.3 76.3 85.4 95.2++ 79.7 64.1 _(300° C.)x10⁷C⁻¹ *comparative example**α from 25° C. to 140° C. +α from 25° C. to 240° C. ++α from 25° C. to260° C.

Examples 12-19

Examples 12 to 19 are modifications of the zinc niobium phosphate oftable 1. Most of these glasses (examples 12 to 18) were prepared inAl₂O₃ crucible an the source of P₂O₅ was NH₄ H₂ PO₄. Melting in Al₂O₃crucible increases the Al₂O₃content of the glass and modifies thephysical properties. The compositions of table 2 were calculated frombatch. Example 19 was melted in platinum crucible and the source of P₂O₅was H₃ PO₄. Example 12 is the same composition as example NO. 9 of table1, comparison shows that melting in Al₂O₃ crucible (example No.12)increases the Tg and Td and lowers the expansion coefficient. Example 13(the same composition as example No. 10 of table 1) shows that theviscosity increases due to the melting in Al₂O₃ crucible and even at1250° C. the glass is very viscous to pour. Similar behavior is shown bythe Ta₂O₅ containing glass (example No. 14). Comparison of examples 12and 14 shows that Ta₂O₅ raises the Tg and Td and hence the viscositycompared to Nb₂O₅. Example No. 14 had blue color like the niobiumcontaining glasses. Examples 15 to 18 show the beneficial effects ofBi₂O₃ , Al₂O₃ and Na₂O: low Tg and Td and fairly small linearcoefficient of expansion.

Example No. 19 shows that by proper selection of the ingredients andmelting in platinum crucible very good properties are possible.

All the glasses of table 2 have very good durability.

12 13 14 15 16 17 18 19 ZnO 30 30 30 32 35 32 32 32 P₂O₅ 65 62.5 65 5555 56.99 55 62 Nb₂O₅ 5 7.5 — 5 0.01 0.01 5 3 Al₂O₃ — — — 4.99 5 — 3 Na₂O— — — 3 3 3 3 — Bi₂O₃ — — — 5 5 — 5 — Ta₂O₅ — — 5 — — — — — CrucibleAl₂O₃ Al₂O₃ Al₂O₃ Al₂O₃ Al₂O₃ Al₂O₃ Al₂O₃ Pt Tg (° C.) 403 * ** 386 406451 357 360 Td (° C.) 464 * ** 448 453 505 414 437 α₁₀₀₋  84.6 — —  79.5 94.5  76.2  85.7  85.6 _(300° C.)x10⁷C⁻¹ *too viscous to pour at 1250°C. **too viscous to pour at 1400° C.

1. A glass composition consisting of, in terms of mole percent of thetotal composition, a. 40-65% P₂O₅; b. 0.1-15% Nb₂O₅; c. 25-37% of ZnO;d. 0-6% Al₂O₃; e. 0-10% Bi₂O₃; f. 0-5% B₂O₃ g. total of 0-5% of Ta₂O₅,V₂O₅, Sb₂O₃, ZrO₂, and/or TiO₂; h. 0-3% alkali oxides; i. 0-5% alkalifluorides; and j. total of 0-5% oxides of Mg, Ca, Sr, Ba, Cu, Fe, and/orMn.
 2. A glass composition according to claim 1, comprising Al₂O₃.
 3. Aglass composition according to claim 1, comprising Bi₂O₃ and/or B₂O₃. 4.A glass composition according to claim 1, comprising 0.1-3% Na₂O.
 5. Aglass composition according to claim 1, including at least 50% P₂O₅. 6.A glass composition according to claim 1 comprising up to 10% Nb₂O₅. 7.A glass composition according to claim 1, having T_(g) of between 200°C. and 500° C.
 8. A glass composition according to claim 1 or 7, havingan expansion coefficient of 12×10⁻⁶ K⁻¹ or less.
 9. A glass compositionof claim 1 that is free of lead and cadmium, has a T_(g) of between 200°C. and 500° C., and expansion coefficient of 12×10⁻⁶ K⁻¹or less, saidglass comprising, by mol percent of the total composition, at least 3%Nb₂O₅ and/or Al₂O₃, 25-37% ZnO, and at least 40% P₂O₅.
 10. A glasscomposition according to claim 9, having an expansion coefficient of10×10⁻⁶ K⁻¹ or less.
 11. A glass composition according to claim 9,comprising 3-10% Nb₂O₅.
 12. A glass composition according to claim 9,comprising 3-6% Al₂O₃.
 13. A glass composition according to claim 9,which is further free of nickel, chromium, and arsenic.
 14. A glasscomposition according to claim 9, comprising no more than a total of 3%of alkali oxide, alkali fluoride, and mixtures thereof.
 15. A thick filmformulation comprising particles of glass according to claim 1 or 9,dispersed in an organic medium.
 16. A thick film formulation accordingto claim 15, further comprising solid particles other than saidparticles of glass.
 17. A thick film formulation according to claim 16,wherein said solid particles are conducting particles and/or ceramicfillers.
 18. A thick film formulation according to claim 16, wherein thesolid particles include 0.5-80% (w/w) of glass particles.
 19. A thickfilm formulation according to claim 18, comprising up to 20% of ceramicfillers.
 20. An article comprising a thick film formulation according toclaim 15 fired on a ceramic substrate.
 21. A glass compositionconsisting of, in terms of mole percent of the total composition, a.40-65% P₂O₅; b. 0.1-15% Nb₂O₅; c. 25-37% of ZnO; d. 0-6% Al₂O₃; e. Bi₂O₃in an amount greater than 0 up to 10%; f. 0-5% B₂O₃ g. total of 0-5% ofTa₂O₅, V₂O₅, Sb₂O₃, ZrO₂, and/or TiO₂; h. 0-3% alkali oxides; i. 0-5%alkali fluorides; and j. total of 0-5% oxides of Mg, Ca, Sr, Ba, Cu, Fe,and/or Mn.
 22. A glass composition consisting of, in terms of molepercent of the total composition, a. 40-65% P₂O₅; b. 0.1-15% Nb₂O₅; c.25-37% of ZnO; d. 0-6% Al₂O₃; e. 0-10% Bi₂O₃; f. 0-5% B₂O₃ g. greaterthan 0 of V₂O₅ and/or Ta₂O₅, and optionally one or more of Sb₂O₃, SRO₂and TiO₂, in a total amount of up to 5%; h. 0-3% alkali oxides; i. 0-5%alkali fluorides; and total of 0-5% oxides of Mg, Ca, Sr, Ba, Cu, Fe,and/or Mn.
 23. A glass composition consisting of, in terms of molepercent of the total composition, a. 40-65% P₂O₅; b. 0.1-15% Nb₂O₅; c.25-37% of ZnO; d. 0-6% Al₂O₃; e. 0-10% Bi₂O₃; f. 0-5% B₂O₃ g. greaterthan 0 of Ta₂O₅, and optionally at least one of V₂O₅, Sb₂O₃, ZrO₂ andTiO₂ in a total amount of up to 5%; h. 0-3% alkali oxides; i. 0-5%alkali fluorides; and j. total of 0-5% oxides of Mg, Ca, Sr, Ba, Cu, Fe,and/or Mn.